Fill a bowl with hot water and hold the shaker upside down with just the cap in the water. Warning - use a towel to prevent burning your hands because the metal will be hot. This should work because metal expands a bit with heat and that may expand it enough to be able to remove.

If this still doesn't work, when the cap is still hot, stick an ice pick or screwdriver (with a cloth on the end to protect the cap) through one of the strainer holes, then tap gently with a hammer to pop off the cap.